Carbonated soft drinks are manufactured by combining soft drink concentrate, cold water, and carbon dioxide and then packaging the composition in bottles or cans. The filled container will be transported away from the filler on automated conveyors, may have a label applied, will be inserted into secondary packaging which can be crates, polymer rings, paperboard cartons, or shrink wrapped trays, and finally will be assembled into palletized loads ready for storage and shipping. During handling and transportation from the filler to the final palletized form, containers frequently come into contact with aqueous compositions such as rinse water and water based conveyor lubricants. As used herein, “aqueous composition” refers to compositions that comprise greater than about 90% by weight of water, and includes water, treated water, and water to which one or more functional ingredients have been added. Treated water includes water that has been processed to improve some quality of the water, for example water processed to reduce the concentration of impurities and dissolved materials or to reduce the concentration of viable microorganisms. “Aqueous composition” includes, but is not limited to bottle rinse water, bottle warmer water, case washer water, and lubricant compositions having water as part of the composition. Because containers are filled at high rates up to and exceeding thousands of containers per minute, some spilling of beverage is likely, especially in the case of carbonated beverages which may foam. Containers frequently will be rinsed immediately downstream of the filler to remove spillage. Because containers are filled with ice cold beverage, it is typically required to rinse them with a warm water rinse in order to raise the temperature of their contents to a value above the dew point, thereby minimizing condensation inside secondary packaging such as boxes or shrink wrap enclosures. Therefore, containers will usually be rinsed a second time in a so called bottle warmer or can warmer. To facilitate rapid movement of beverage containers at speeds up to thousands of containers per minute and higher, it is conventionally required to apply lubricant compositions to the bottle or conveyor surfaces.
Preferred containers for carbonated soft drinks are thermoplastic bottles made from polyethylene terephthalate (PET). Polyester resins including PET are hydrolytically susceptible, meaning they can react with water in a process known as hydrolysis. The word hydrolysis comes from the Latin roots “hydro” and “lyso” meaning to break apart with water. In this process, water reacts with PET to create two new chain ends and the PET polymer chain is cut. Polymer under stress is much more reactive with regard to hydrolysis, and amorphous PET like that found in parts of carbonated soft drink bottles is more susceptible to hydrolysis than crystalline or oriented PET. Polymer residing at the bottom of carbonated soft drink bottles near the “gate scar” is under stress from the carbonation and is also amorphous, so it is particularly susceptible to hydrolysis. The gate scar is a round bump in the center of the bottom of every PET beverage bottle which is an artifact of the bottle manufacturing process. In the first step of the bottle manufacturing process, a test tube shaped “preform” that will later be heated and inflated inside of a mold of the final bottle shape is made by injection molding. When PET is forced into the preform injection mold through a gate, a short stem of PET remains attached to the bottom of the test tube, and when this stem is cut, the gate scar is left as a round stub. The gate scar is typically withdrawn slightly from bottom most portion of a PET beverage bottle, and for a bottle standing on a flat table, the gate scar will be about 0.05 to 0.15 inches above the table surface.
Hydrolysis of PET in carbonated soft drink bottles that proceeds to the point of bottle failure is known as “stress cracking.” By failure, it is meant that one or more cracks propagate through the wall of the bottle and there is a loss of liquid contents. Bottles which fail by stress cracking and bottles nearby which are wetted with spilled beverage become unsellable, and stress cracking can lead to substantial losses of merchandise and productivity.
The problem of stress cracking in PET bottles filled with carbonated soft drinks has not been well understood. Many investigations have studied stress cracking indirectly. That is, instead of measuring the relative rate of failure, they have measured some property that is believed to correlate with the tendency towards failure such as appearance, time to failure, or rupture stress for samples in contact with chemical compositions. For example, it has been assumed that the appearance of PET beverage bottle bases after exposure to a test composition is an indication of the extent of bottle failure that will occur if bottles contact the test composition in production. However, it can be seen from the Examples below that there is essentially no correlation between the bottle failure rate and that bottle appearance (as quantified by a crazing score) that results from contacting PET bottles with test compositions. Another test used to predict bottle failure rates is the International Society of Beverage Technologists (ISBT) Accelerated Stress Crack Test Method. According to this test, bottles are exposed to sodium hydroxide solution, and the exposure time required to cause the bottle to fail is recorded. In variations of this test, other chemicals have been added to the sodium hydroxide solution. Another indirect test is ISO 6252: 1992(E), “Plastics—Determination of environmental stress cracking (ESC)—Constant-tensile-stress method” available from the International Organization for Standardization (ISO). In the ISO 6252 test, polymer strips are subjected to a constant tensile force corresponding to a stress lower than the yield stress while submerged in a test liquid, and the time or stress at which the strip breaks is recorded. It has often been preferred to use one of these or other indirect test methods to predict failure rates rather than measure failure rates of bottles directly. Indirect test methods are relatively simpler and less expensive to conduct. However, there is growing awareness that indirect tests have overall poor correlation to actual bottle failure rates and that many conclusions about the PET “compatibility” of chemical compositions based on indirect testing are incorrect. Preferably, PET “compatibility” is determined directly by measuring the actual failure rate of bottles in conditions similar to those in bottle filling and storage, for example by using the PET Stress Crack Test described below.
Hydrolysis of ester bonds which form the linkages in PET chains is known to be catalyzed by bases, so it is logical to assume that alkalinity in aqueous compositions that contact PET bottles should be avoided. The conclusion from much testing and experience is that alkalinity in aqueous compositions is indeed a key factor in PET bottle stress cracking. However, a guideline for the permissible levels and types of alkalinity is not generally agreed upon. Three naturally occurring types of alkalinity in water sources are hydroxide alkalinity, carbonate alkalinity, and bicarbonate alkalinity. Generally, bicarbonate alkalinity is the most common type of alkalinity found in water sources, while hydroxide alkalinity is usually absent or present at relatively insignificant levels of less than one percent of the total of hydroxide plus bicarbonate plus carbonate alkalinity. The sum of hydroxide, carbonate, and bicarbonate alkalinity of water which is allowed to contact PET bottles in bottling plants typically ranges between about 10 ppm and 100 ppm, expressed as ppm of CaCO3 (calcium carbonate), with occasional values above 100 ppm. On the other hand, the International Society of Beverage Technologists (ISBT) web site strongly recommends to keep the total alkalinity level (expressed as CaCO3) below 50 mg/L (equivalent to 50 ppm) in all water that could potentially contact the bottle (including, but not limited to: lube makeup water, rinser water, warmer water, case washer, etc) in order to minimize the risk of stress crack failure. When tested using the PET Stress Crack Test in the examples section, water within the ISBT guideline containing 50 ppm or even 25 ppm of bicarbonate alkalinity (expressed as CaCO3) will still give significant amounts of failure, in comparison to deionized or distilled water, which will give no failure.
There have been two main approaches to minimizing the risk of stress cracking due to alkalinity. One approach has been to purify water that comes in contact with PET bottles, and the other has been to use a conveyor lubricant composition that mitigates the effect of water alkalinity.
Purification of water that contacts PET bottles may be done using processes including ion exchange, lime/lime soda softening, split stream softening, and membrane separation processes such as reverse osmosis and nanofiltration. Although the approach of purifying water that contacts PET bottles has proven to be very useful industrially for reduction of stress crack incidents, bottle failure has a strong dependence upon alkalinity even at very low levels and there is uncertainty about what are meaningful specifications for purified water that will provide an acceptable reduction in risk for stress crack failure. Stress cracking has a strong dependence on other environmental variables such as temperature and humidity, and due to the many factors involved in PET stress cracking, it is impossible to determine a single “safe” alkalinity level for aqueous compositions that contact carbonated PET bottles. For example, when PET bottles filled with carbonated liquid are stored under conditions of high temperature and high humidity, bottles that contact water with the ISBT recommended limit of alkalinity at 50 ppm as CaCO3 will exhibit significantly more failure than bottles that have only contacted deionized or distilled water. Alkalinity is not monitored and controlled in all bottling facilities and in case the alkalinity level increases (for example due to equipment failure) it is beneficial to have other means for mitigating the risk of alkaline induced stress cracking.
One way to counteract the effects of alkalinity has been through the use of conveyor lubricant compositions, specifically foaming conveyor lubricants. Conveyor lubricant compositions can be effective to erase the effects of alkalinity in the lubricant composition itself and alkalinity that contacted the bottle in a previous rinse step. For a conveyor lubricant composition that mitigates water alkalinity to be effective at reducing failure due to residual alkalinity from bottle rinsing, it must be applied to bottles downstream of the point of application of the rinse. Effective application of a conveyor lubricant downstream of rinsers and warmers requires the lubricant to contact the susceptible gate scar region. Because this region rides about 0.05 to 0.15 inches above the conveyor surface, in order for the lubricant to make contact, it must be sprayed directly on each bottle or it must be of sufficient depth on the conveyor. In practice, sufficient depth of the lubricant composition on conveyors downstream of rinsers and warmers is provided by foaming the lubricant. In this case, the lubricant must have a tendency to foam. The tendency of a lubricant to foam can be determined using a Foam Profile Test as described below. According to this test, non-foaming lubricants have a foam profile less than about 1.1, moderately foaming lubricants have a foam profile between about 1.1 and 1.4, and foaming lubricants have a foam profile value greater than about 1.4. An example of a foaming conveyor lubricant which works well under conditions of high alkalinity in water sources is LUBODRIVE RX, available from Ecolab, St. Paul, Minn. The foam profile for one part of LUBODRIVE RX diluted with 199 parts of 168 ppm sodium bicarbonate solution is 1.6. Non-foaming lubricants have generally not been used with stress crack susceptible PET packaging in the case that aqueous rinse compositions contain greater than about 50 ppm alkalinity as CaCO3 because of the inability to reach the gate scar region of the bottle.
Newer and particularly preferred conveyor lubricants including silicone emulsion based lubricants are non-foaming. Non-foaming silicone based lubricants will not contact the gate portion of the bottle and some other means is required to lessen the risk of stress cracking resulting from contact of bottles with aqueous rinse compositions that contain alkalinity. Silicone based lubricants are preferred lubricants for PET bottles because they provide improved lubrication properties and significantly increased conveyor efficiency. Silicone containing lubricant compositions are described, for example in U.S. Pat. No. 6,495,494 (Li et al which is incorporated by reference herein in its entirety). Particularly preferred conveyor lubricants are “dry” lubricants as described in U.S. patent application Ser. No. 11/351,863 titled DRY LUBRICANT FOR CONVEYING CONTAINERS, filed on Feb. 10, 2006 which is incorporated by reference herein in its entirety. Dry lubricants include those that are dispensed onto conveyors in a neat undiluted form, those that are applied to the conveyor intermittently, and/or those that leave the conveyor with a dry appearance or are dry to the touch. In the case of dry lubricants, the lubricant will not contact the stress crack susceptible gate portion on the majority of bottles processed.
U.S. patent application Ser. No. 11/233,596 titled SILICONE LUBRICANT WITH GOOD WETTING ON PET SURFACES, filed on Sep. 22, 2005 and U.S. patent application Ser. No. 11/233,568 titled SILICONE CONVEYOR LUBRICANT WITH STOICHIOMETRIC AMOUNT OF AN ORGANIC ACID, filed Sep. 22, 2005 both of which are incorporated by reference herein in their entirety, describe silicone conveyor lubricant compositions that exhibit improved compatibility with PET. While additives described in U.S. patent application Ser. No. 11/233,596 and Ser. No. 11/233,568 represent substantial improvements over prior art compositions, they may impart properties to lubricant compositions that are in some cases undesirable. For example, additives described in U.S. patent application Ser. No. 11/233,596 and Ser. No. 11/233,568 may modify the lubrication properties and may result in a pH for the composition that is low relative to compositions without additives. If added in large amounts, addition of components to improve PET compatibility as described in U.S. patent application Ser. No. 11/233,596 and Ser. No. 11/233,568 may result in reduced stability of the resulting composition in the case that the composition comprises an emulsion. Therefore, there exists an opportunity for improving the combination of PET compatibility and other properties of silicone based conveyor lubricants.
Although much progress has been made in reducing the incidence of stress cracking, every year incidents still occur. While opportunities exist for reducing the risk of stress cracking, there is increasingly a greater need to do so. The beverage industry is characterized by relentless changes including new beverage products, new bottle designs, cost and waste reduction, and faster and more efficient manufacturing processes. It is important that as changes occur, the risk or incidence of stress cracking does not increase.
The rising cost of petrochemicals, including raw materials used to make PET creates an incentive to minimize the amount of PET in every beverage bottle. The practice of minimizing the amount of PET used in a beverage bottle design is called lightweighting. Increased cost of petrochemicals will also provide motivation to use polymers from renewable sources such as agricultural feedstocks. Poly(lactic acid) (PLA) is derived from agricultural sources and like PET, is a polyester that can hydrolyze with water. Improving the compatibility between aqueous compositions used during filling and conveying of bottles and hydrolysis susceptible polymers can facilitate the practice of lightweighting, allow a reduction in the mass of polymer used per bottle, and facilitate the use of new polymers including those derived from renewable sources.
There is also an incentive to use recycled PET as a feedstock for manufacturing of beverage bottles. Unlike many other polymers, the molecular weight of PET can be upgraded during the recycling process, improving the properties of the polymer which may have degraded in previous fabrication and use. However it is well known that processing of PET including injection molding of preforms and blowing preforms to give bottles results in degradation of properties including diminution of molecular weight. Furthermore, post consumer recycled (PCR) PET may include other resins, polyester resins other than PET such as glycol modified PET (also known as PETG or poly ethylene terephthalate glycol copolyester), and impurities such as colorants, catalysts, and remnants of active and passive barrier materials. Increasing the amount of PCR PET in beverage bottles may result in increased risk of bottle failure due to stress cracking. However, a greater PCR polymer content in beverage bottles may be allowable by improving the PET compatibility of aqueous compositions that contact PET bottles during filling and conveying.
For reasons including extending shelf life, allowing smaller package size, improved product quality and allowing lighter bottles, there is motivation to use barrier layers in PET bottles which minimize the egress diffusion of carbon dioxide and ingress diffusion of oxygen. Active barrier materials are those that react with the diffusing species, and passive barriers are those that impede the diffusion of the diffusing species without reaction. While externally applied barrier layers can potentially provide a layer of protection for the underlying PET, use of barrier layers can also increase susceptibility towards stress cracking. For example, barrier layers will generally allow the use of lighter weight bottles. Barrier layers which slow the egress diffusion of carbon dioxide can allow a higher pressure differential to be maintained between the inside and outside of the bottle resulting in greater tensile stress on the bottle wall, and may diminish the concentration of carbon dioxide at the exterior surface of the PET bottle wall, effectively raising the local pH and thereby increasing the rate of hydrolytic degradation of PET. Improving the PET compatibility of aqueous compositions which contact bottles may be important to diminish the incidence of stress cracking in PET bottles which comprise a barrier layer.
It is against this background that the present invention has been made.